A tantalizing trio of Earth-size worlds circles a tiny, dim star relatively close to us, and each planet is within or near the region where the star’s light could support the whispers and sighs of extraterrestrial life.
But don’t get too hopeful for Earth look-alikes—while we don’t know much yet about their characteristics, these three worlds exist in very different environments than our home planet.
Still, this is the first time three such worlds have been spotted around an ultracool dwarf star, a discovery that bodes well for planet-hunters scouring the galaxy for small, rocky planets.
“This is a brand new planetary population that is revealed, and it could well dominate the total number of planets in the Milky Way,” says Michaël Gillon of Belgium’s University of Liège. “This indicates that the formation of Earth-size planets around these downplayed tiny stellar objects—that are much more frequent than sunlike stars in the galaxy—is very efficient.”
As they report this week in Nature, scientists found the system using the Transiting Planets and Planetesimals Small Telescope, or TRAPPIST. This robotic instrument in Chile searches for planets around the 60 brightest ultracool dwarfs.
The host star for the newfound trio, called TRAPPIST-1, is just a smidge bigger than the planet Jupiter, and it is so cool that most of the light it emits is in the infrared. So, if alien life happened to emerge on any of the star’s three known planets, the landscapes might look very different from our green world.
On Earth, photosynthesis is based on pigments like chlorophyll, which absorbs blue and red light and appears green to human eyes. But on the TRAPPIST-1 planets, chemistry based on visible light would be pretty inefficient, Gillon says.
“It is thus a priori likely that evolution would favor the development of infrared-sensitive pigments for photosynthesis,” he says. Instead, these alien plants would absorb the infrared and tiny bits of red light the star emits—and might appear rather lackluster and black to human eyes.
“Of course, for local creatures with infrared vision, plants would have some colors and would look much nicer,” Gillon says.
At just under 40 light-years away, the system is close enough to make further study of its characteristics plausible in the very near future. Scientists are keen to do that, because the cosmos doesn’t offer us a handful of Earth-size worlds in the near neighborhood very often.
Two of the planets, TRAPPIST-1b and TRAPPIST-1c, are just a bit too close to their star to be racing around inside the habitable zone, the region where liquid water can stably exist on the surface.
A year on TRAPPIST-1b flashes by in just 1.5 Earth days. And at 2.4 Earth days, a year on TRAPPIST-1c is only slightly more leisurely. Because they’re so snuggled up to the star, the planets are likely orbiting in lockstep, with one face perpetually pointed toward their home star. That means their continually day-lit hemispheres are probably much too toasted for life to comfortably thrive.
Meanwhile, the planets’ dark backsides are also probably inhospitable, although there may be a region between those two extremes that could be just right for life.
The third planet, TRAPPIST-1d, is perhaps the most scientifically intriguing. Its orbit hasn’t been definitively pinned down, and a year there could last anywhere between 4.5 and almost 73 Earth-days.
Based on the team’s observations, there’s a decent chance this planet sits in that sweet spot where life might thrive. If not, it’s plausibly a frozen world, but one where metabolic furnaces could be churning away in a warmer subsurface.
Ice and Air
TRAPPIST found all three worlds by noting when the planets passed between the star and Earth, briefly dimming the star’s light. From the timing and amplitude of those intermittent twinkles, the team could determine their sizes and orbits.
However, this method of planet hunting can’t tell us everything about the system, including how massive the planets are. Without that measurement, it’s tricky to say what the three worlds are made of.
For now, the team suspects the worlds are solid and possibly icy, perhaps somewhat of a hybrid between Earth and the icy moons of the outer solar system, like Europa or Enceladus.
“We don't have such planets in our own solar system, so it is really interesting to find out what such planets can be like,” says Cornell University’s Lisa Kaltenegger, who isn’t involved in the study. “Whether life would thrive under the surface of a frozen world, and if it could have started in the first place, are the open questions that make our search in general that interesting.”
Because the system is so close by, we may soon be able to gather more clues, perhaps by measuring the planets’ atmospheres. That alone can tell us much more about their chances for supporting life.
Astronomers have already pointed the infrared eye of the Spitzer Space Telescope at TRAPPIST-1, and this week scientists plan to place the system in the Hubble Space Telescope’s crosshairs to take a closer look at the two inner planets’ atmospheres.
Over the next decade, as the tools that scrutinize the cosmos become more powerful, scientists could even search the worlds for the signatures of molecules associated with life, such as ozone, oxygen, carbon dioxide, and methane.
“What will matter is how abundant they are in the conditions present in these atmospheres,” says study coauthor Julien de Wit of MIT. “For example, [molecular oxygen] on Earth is present in an extremely high fraction—more than a chemical equilibrium would allow for.”
If aliens are out there looking at Earth, that abundance of atmospheric oxygen would be a clue that we exist. So for people hoping to find life elsewhere in the cosmos, de Wit says, “searching for such a chemical disequilibrium is the main path forward.”